This invention relates to power amplifiers and more particularly to bridge amplifiers.
As is known in the art, power amplifiers have a wide range of applications. One such power amplifier is a bridge amplifier. An early bridge amplifier is described in U.S. Pat. No. 2,235,677 Entitled xe2x80x9cAmplifier for Signal Transmissionxe2x80x9d issued Mar. 18, 1941, inventor S. Gubin. The bridge amplifier described in the Gubin patent used thermionc emission devices. With the advent of transistors, the amplifier of Gubin was further improved by including a feedback signal to sense mismatches in the transistors and thereby apply a compensation voltage to the control electrodes of the transistors and thereby maintain apparent identity in operating characteristics of the transistors. Such improvement is described in U.S. Pat. No. 3,157,839, issued Nov. 17, 1064, entitled xe2x80x9cTransistor Bridge Amplifier with a Bias Compensation Circuit Thereforexe2x80x9d, inventor H. B. Brown et al. Further improvement in the bridge amplifier were made by using, inter alia, a pair of feedback signals, one proportional to current passing through the load connected to the output of the amplifier and one proportional to the voltage produced across the load. Such improved bridge amplifier is described in U.S. Pat. No. 4,092,610, issued May 30, 1978 entitled xe2x80x9cModulated Carrier Amplifying Systemxe2x80x9d, inventors Benjermin J. White, George Moreau, Robert E. Dworkin, assigned to the same assignee as the present invention.
As described in U.S. Pat. No. 4,092,610, a signal to be amplified, after being combined with the feedback signal, is modulated with a carrier signal and the resulting signal is fed to the bridge amplifier. One such amplifier circuit 10 is shown in FIG. 1. Thus, the input signal is fed to a summing amplifier 12 analog with a pair of feedback signals. One such feedback signal VFB1 is a function of the voltage, VL, produced across output terminals 14a, 14b of a bridge amplifier 13 and the other feedback signal VFB2 is a function of the current IL, to the output terminals 14a, 14b. The output of the summing amplifier 12 is combined with the modulating signal, here, for example a square wave of sawtooth wave voltage, produced by carrier signal generator 15, in a pair of comparators 16a, 16b, as shown. The comparators 16a, 16b produce a pair of pulse width modulated (PWM) switching signals PWM1 and PWM2, respectively. Inverters 18a, 18b are provided to generate a complementary pair of pulse width modulated signals PWM1xe2x80x2 and PWM2xe2x80x2, respectively. The four PWM signals are fed to the switching bridge amplifier 13, as shown. The switching bridge amplifier 13 includes four amplifier elements, here transistors 22a, 22b, 22c and 22d arranged as shown in a bridge circuit having four nodes N1, N2, N3, and N4. Nodes N1 and N2 arc connected across a voltage source +Vs and nodes N3 and N4 are connected to output terminals 14a, 14b. The output terminals 16a, 16b are connected to load here represented by a resistor RL, in a manner to be described in more detail below. Switching diodes 24a, 24b, 24c and 24d are arranged as shown. The operation and further details of the circuit shown in FIG. 1 are provided in the above referenced U. S. Pat. No. 4,092,610, referred to above, the entire contents thereof being incorporated herein by reference.
As described in U.S. Pat. No. 4,092,610, the bridge amplifier 13 further comprises a transformer 17 having an input winding coupled via a capacitor to a pair of opposite nodes, here nodes N2 and N4 of the bridge circuit. A current sensor 19 is also coupled in series with the input winding for providing a voltage on line 21 having a magnitude proportional to the current in the input winding. The current sensor 19 comprises a transformer having a single turn of wire as its input winding and a multiple turn winding for its output winding with a potentiometer coupled across the terminals of the output winding. The current sensor 19 presents negligible impedance to the circuit of the input winding. The transformer 17 has a winding, which couples the signal of the winding via the lines 23 to a filter 25. The winding provides a voltage between lines 23, which is proportional to the voltage produced by the winding. The voltage on the lines 21 and 23 provide feedback signals which are summed together and with the input signal, in summing amplifier 12, as described above.
As noted above, the output terminals 14a, 14b are coupled, via transformer 17, to the filter 25. The filter 25 comprises, by a way of example, a resonant tank circuit including an inductor and a capacitor, and a capacitive shunt, which includes a resistor and a capacitor. The tank resonates at the repetition frequency of the carrier frequency signal produced by generator 15. The tank circuit thus blocks the passage of signals at the repetition frequency while the shunt shorts out harmonics of the input signal thereby providing a stop band which minimizes the appearance of carrier and harmonic frequency components at the load, here a transducer, or projector. A capacitor in series with the tank blocks direct current from the load. The load is shown as a resistive element, RL, which coats with the filter to provide the foregoing pass band.
In operation, when amplifying units (i.e., transistors 22a, 22d) conduct current simultaneously, the amplifying elements (i.e., transistors 22b, 22c) conduct no current. Conversely, when the amplifying elements (i.e., transistors 22b, 22c) are driven by switching signals to conduct current simultaneously, amplifying elements (i.e., transistors 22a, 22d) conduct no current. Thus, the direction of current in the input winding of the transformer 17 alternates in direction to provide both positive and negative pulses of current to the load, RL. In view of the filtering provided by the filter 25 and any reactance in both the load, RL and the elements of the bridge amplifier 10, the signal appearing in the load, RL has an instantaneous amplitude which follows that of the input signal fed to the amplifier system 10. The use of the four transistors in the bridge amplifier 13 permits application of a greater amount of power to the load than could be provided by simply one or two transistors. Further, the sequential operation of the branches of the bridge providing for a lower average value of power dissipation in each one of the four transistors.
One application of the amplifier system 10 shown in FIG. 1 is in a sonar system. Such a system is shown in FIG. 2. Here, a digital waveform generator 30 produces digital samples of acoustic wave energy. The digital samples are fed to a transmit bean forming network 32. The beam forming network has a plurality of output ports, each producing the digital samples representing a burst of acoustic wave energy with a predetermined time delay in accordance with the desired transmit direction of the burst of energy. That is the beam forming network 32 is used to provide a collimated and directed beam of acoustic energy. The digital samples at each one of the output ports is fed to a digital to analog (D/A) converter 34, as shown for conversion onto analog (i.e., time-continuous) bursts of acoustic energy. These bursts of energy are fed to a corresponding one of a plurality of the amplifier systems 10 shown in FIG. 1. The amplified bursts of acoustic energy are fed to selected sets of transducers, or projectors 36 in an array of such projectors 36 through an aperture switching network 38, as shown.